that is because CA law says that something can be a "short barreled shotgun" without it first being a shotgun. It is codifed in law.

you don't. that is why you initially build in in a dimensionally-compliant single-shot configurarion.

those that are actually building SSE pistols know about the barrel and OAL requirements.

why would I want to build a single-shot 1911. maybe so that it is built the way I want and that I can be proud that I built my own gun. nothing says that it has to stay as a single-shot. Once I comply with the law and use the exemption afforded to SSE handguns, nothing says that I can't latter modify it into a semi-auto config. Just as if I had bought it from an FFL as an SSE and them converted it afterwards.